1. Field of the Invention
The invention relates to spherical particles on the basis of metal oxides, and to a method for its manufacture and its utilisation.
2. Brief Description of Relevant Art
Spherical particles on the basis of metal oxides are widely used in a variety of industrial fields. Thus they are used for example for manufacturing bulk materials, coatings, films or fibres, to produce optical, electro-optical or opto-electronic components, in chromatography, as fillers or as carriers e.g. for pharmacological active ingredients.
In many fields, the suitability of spherical particles depends, in large part on particle size, particle size distribution and surface quality. Surface quality, for example, is important in situations when the particles are to be used as fillers, when they are introduced into a matrix or when they are used as carriers or in chromatography. Efforts have been made to develop methods for manufacturing and surface modification of spherical particles. The resulting particles have a narrow particle size distribution. In such situations, the particle size can be deliberately controlled and adapted to the respective requirements of the particular end use. Similarly, the surface qualities can be modified in a controlled manner.
Furthermore, when using the spherical particles as fillers, i.e. after intermingling them in a matrix, efforts have been made to precisely adjust the more variable composition parameters, such as the modulus of elasticity, refractive index, impact resistance, X-ray opacity, coefficient of thermal expansion, permeability, etc. There are also requirements for improved inorganic fillers for use in polymeric or polymerisable systems, particularly when embedded in compounds for optical, electro-optical, and opto-electronic components.
Inorganic fillers are known from DE 42 19 287 A1, which teaches monodisperse non-porous, sphere-shaped particles based on SiO.sub.2, TiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.3, V.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or mixed systems thereof. This reference also discusses surface modification by means of covalently bonded organic groups.
A disadvantage in the fillers as disclosed in DE 42 19 287 A1 is that these fillers lack a satisfactory degree of X-ray opacity. In many situations however an excellent refractive index and good X-ray opacity are important. An unfavourable factor in these fillers is that a maximum of 25% of filler can be introduced. Because of this, the characteristics of the material are substantially determined by the matrix and not by the filler. This results in a relatively high degree of shrinkage and thermal expansion.
When materials such as these are used as dental material, it is advantageous that the fillers used should have a contact toxicity which does not exceed specified predetermined limits, as is the case with amalgam. This cannot be achieved with previously known materials and compounds of such materials.